The formation of a contact for a semiconductor structure is a relevant process in fabricating the semiconductor device. For example, contact resistance significantly affects the performance characteristics of the corresponding semiconductor device. As a result, achieving a low contact resistance is typically desired. Other desirable characteristics include contact stability, surface morphology, reliability, and linearity, especially at large currents.
In order to achieve a low contact resistance for Gallium Nitride (GaN) devices, several contact metals and a relatively high annealing temperature are typically utilized to form the contact. Aluminum (Al) is widely used in ohmic contacts because of its low melting point of 660 degrees Celsius. Additionally, Titanium (Ti) or Chromium (Cr) is used as the first layer for the contact because of its low metal work function to GaN. For example, one approach forms a Ti/Al-based contact to an n-type GaN semiconductor having several metals, such as Ti/Al/Ti/Gold (Au) or Ti/Al/Nickel(Ni)/Au, with a thickness ranging from five nanometers to five microns, and which is annealed at 400 degrees Celsius or more. A different approach reverses the order of Ti and Al, and forms an Al/Ti-based contact to an n-type GaN semiconductor that includes Al/Ti/Platinum(Pt)/Au and is annealed at temperatures between 400 and 600 degrees Celsius. Other approaches form a Cr/Al-based contact to an n-type GaN semiconductor that include various metal configurations, such as Cr/Al/Cr/Au, Cr/Al/Pt/Au, Cr/Al/Lead(Pd)/Au, Cr/Al/Ti/Au, Cr/Al/Cobalt(Co)/Au, and Cr/Al/Ni/Au.
To date, a Ti/Al-based contact yields a lower contact resistance than a Cr/Al-based contact, while requiring a higher temperature annealing for the contact alloy. However, the Ti/Al-based n-contact is not reliable for 265 nanometer (nm) and shorter wavelength ultraviolet (UV) light emitting diodes (LEDs). The Cr/Al-based contact has a lower annealing temperature, but a higher contact resistance, than the Ti/Al-based contact. Because of the lower annealing temperature, the surface morphology of the Cr/Al-based contact is better than that of the Ti/Al-based contact.
Various research has been devoted to improving the Ti/Al-based contacts and the Cr/Al-based contacts. In one approach, a Cr/Ti/Al-based contact with continuous layers was proposed.